We are continuing our studies of structures of bacterial iron-containing proteins, functional in photosynthesis and nitrogen fixation, to provide a detailed molecular basis for a comparative biochemistry of iron-containing proteins. As noted in last year's Summary, a number available in pure from and bulk quantities have been crystallized and found adequate for tertiary structure determinations at high resolution, using a variety of procedures including x-ray crystallographic analysis, and a wide range of modern spectrochemical methodology (ORD, CD, NMR, EPR and IR spectroscopy). Some progress has been made in application of magnetic circular dichroism technology to the particular case of assignment of IR transitions for ferricytochromes c'. Crystallization of several variant cytochromes c has proceeded to the point of initiation of tertiary structure studies by x-ray crystallographic procedures. Amino acid sequences for these proteins have been determined. Complete reports of high resolution x-ray determinations of tertiary structure have been provided in studies by associated groups for R. rubrum cytochrome c2 (reduced and oxidized) and of reduced Chromatium "HIPISP."